Copyright © Glencoe/McGraw-Hill Carpentry & Building Construction Basic Roof Calculations Span Unit run Total run Unit rise Total rise Pitch

Copyright © Glencoe/McGraw-HillCarpentry & Building ConstructionBasic Roof Calculations

Basic Roof Calculations

Span Unit run Total run Unit rise Total rise Pitch


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Safety Notice


Basic Roof Calculations

Observe safety rules.Make sure tools and equipment are

available. Check materials.Follow the steps described here.


Safety

If you are at a worksite, wear all the required safety equipment.


Tools and Equipment

Calculator


Materials

Paper and pencil


Procedure

The steps that follow describe the terms and calculations used for building a roof.


Step 1

The span is the distance between the outside edges of opposing exterior walls.


Step 1

Measure straight across the building, not at an angle.


Step 2

The unit run is used to figure the roof angle. For a common rafter, the unit run is always 12".


Step 3

The total run is one-half the span. Divide the span by two.

Example:If the span is 24´, the total run is:

= 12´242

Exception: This rule does not work when the pitch is different on opposite sides of the ridge board.Exception: This rule does not work when the pitch is different on opposite sides of the ridge board.


Step 4

The unit rise is the number of inches that a roof rises for every 12" of run (unit run).

The triangle above the roof in this drawing shows the unit rise (8) and unit run (12).


The angle of the roof will change if the unit rise changes.

Step 4


Step 5

The total rise is the vertical distance from the top of the top plate to the upper end of the measuring line.

Total rise


Step 5

a. The measuring line is an imaginary line running from the outside wall to the ridge.

Total rise


Step 5

b. The total rise equals the unit rise multiplied by the total run.

15 x 8" = 120"120" = 10´

Total Rise

15'

10'


Step 6

The pitch is the angle that a rafter makes witha horizontal plane.a. Pitch is the ratio of the unit rise to the

unit run.

b. Pitch is written as a fraction:unit riseunit run

c. In a drawing, the triangle symbol shows the pitch visually.


Step 6

Example: In this drawing, the roof has an 8" unit rise and a 12" unit run.

a. The roof has an “eight/twelve pitch” or an “eight in twelve pitch.”

b. The pitch is written as 8/12.

Total rise


Step 7

a. What is the unit rise in the roof shown here?

Answer: 6"


Step 7

b. What is the span?

Answer: 28´


Step 8

What is the pitch of the roof shown here?

Answer: 8/12

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Copyright © Glencoe/McGraw-Hill Carpentry & Building Construction Basic Roof Calculations Span Unit run Total run Unit rise Total rise Pitch